1. Field of the Invention
The present invention relates to film formation using an organic silane type source gas. More specifically, the invention relates to a manufacturing method of a semiconductor device including formation of a film containing hydrogen and nitrogen which film is low in the content of carbon components and superior both in step coverage and impurity blocking performance.
2. Description of the Related Art
In LSIs, which constitute one technical field of semiconductor devices, the wiring interval is now as small as 0.2-0.4 .mu.m and the aspect ratio (height to width) of wiring lines (interconnections) now exceeds unity. To prevent voids from occurring in planarizing an interlayer insulating film, a film forming method comes to be used which utilizes superior step coverage of a film that is formed by using an organic silane type source gas such as ethyl orthosilicate (Si(OC.sub.2 H.sub.5).sub.4, what is called "TEOS"). In another field of liquid crystal displays in which a number of thin-film transistors are formed on an insulative substrate, the frequency of occurrence of what is called a "break at a step" of wiring lines of thin-film transistors is lowered by utilizing superior step coverage of a film formed by using ethyl orthosilicate as a source gas. In particular, in liquid crystal displays using a process of lower than 600.degree. C. in contrast to high-temperature processes for silicon wafers, an ethyl orthosilicate source gas is used to form a gate oxide film and an undercoat film in addition to an interlayer insulating film.
In the field of LSIs, although an oxide film formed by using ethyl orthosilicate is used as an interlayer insulating film, it contains many carbon-hydrogen bonds and oxygen-hydrogen bonds and therefore is high in hygroscopicity. On the other hand, although a silicon nitride film exhibits high water resistance and impurity blocking performance, it is inferior in step coverage and is easily broken because of its high degree of hardness.
In thin-film transistors (TFTs), which are applied to, for instance, a liquid crystal display, an undercoat film, a gate insulating film, an interlayer insulating film, and the like are formed on an insulative substrate such as a glass substrate by thermal CVD, plasma CVD, or a like method by using an organic silane type source gas such as ethyl orthosilicate. However, having a large amount of carbon, such films are not sufficient in terms of water resistance and impurity blocking performance.
In a conventional, commonly employed film forming method that is a plasma CVD method using ethyl orthosilicate, a subject substrate is placed in a chamber having parallel-plate electrodes and capable of being evacuated. On of the electrodes is connected to a high-frequency power supply, that is, serves as the cathode. The other electrode is connected to the ground, that is, serves as the anode. The subject substrate is placed on the ground-side, i.e., anode-side electrode. Since ethyl orthosilicate assumes liquid form in the ordinary temperature, it is introduced into the chamber in a state that it is heated to increase its vapor pressure or it is introduced into the chamber together with a carrier gas by bubbling ethyl orthosilicate in a tank with the carrier gas. Ethyl orthosilicate has a feature that when decomposed in plasma, it forms precursors and flows on the substrate, thus enabling formation of a film that is superior is step coverage. Precursors moving on the substrate collide with each other, and oxygen ions, oxygen radicals, ozone molecules formed in the plasma collide with those precursors, causing abstraction reaction on the surface and thereby forming Sio.sub.x. If a larger amount of oxygen is introduced, the surface abstraction reaction due to the precursors that are formed from ethyl orthosilicate is accelerated. In this case, the step coverage is degraded though the carbon content is reduced.
On the other hand, if a smaller amount of oxygen is introduced, although the step coverage is improved, more carbon-hydrogen and oxygen-hydrogen bonds remain in the film, making it highly hygroscopic. If an infrared measurement is conducted, the absorption in the vicinity of 3,660 cm.sup.-1 will increase with the lapse of time. The absorption at 3,660 cm.sup.-1 is mainly due to Si--OH bonds and indicates that a film formed is hygroscopic.
Another film forming method using ethyl orthosilicate is an atmospheric pressure CVD method utilizing ozone and heat. In this method, a substrate is heated to 300-400.degree. C. An organic silane type source gas such as ethyl orthosilicate is introduced into a reaction chamber by bubbling it in a tank with N.sub.2. Ozone is also introduced into the chamber by generating it by passing oxygen through an ozonizer. Because of superior step coverage and a high film forming rate of this method, this method is used to form an interlayer insulating film for devices that include multi-layer wiring, such as LSIs and DRAMs memories. After the film formation, planarization is performed by etching back, SOG (spin on glass), CMP (chemical mechanical polishing), etc. in combination.
However, according to the above atmospheric pressure CVD method, a resulting film is very low in density, that is, a porous film is formed. Therefore, if such a film is used singly, it exhibits very high hygroscopicity, possibly causing leak between wiring lines, thus lowering the reliability of a semiconductor device. Further, at the present time when the application of the 0.3-.mu.m rules is pressing, the lateral capacitance between wiring lines is not negligible, which requires a film having a small permittivity.
Japanese Unexamined Patent Publication No. Hei. 1-48425 of the present assignee discloses a film forming method that uses an organic silane type source gas and nitrogen oxide. As disclosed in the above publication, this method can form a uniform coating on an uneven surface which coating blocks alkaline impurities. Although this coating functions satisfactorily when used only as an interlayer insulating film, the carbon content in the organic silane type source gas needs to be minimized when this coating serves as an insulating film whose electrical characteristics are important, such as a gate insulating film or a part of a capacitor. The coating cannot be used as an insulating film whose electrical characteristics are utilized unless the carbon content is controlled.
Conventionally, where a film is formed by using an organic silane type source gas such as ethyl orthosilicate, improving the step coverage necessarily causes increase in hygroscopicity and carbon content, which in turn causes reduction in reliability and degradation in semiconductor characteristics. If a large amount of oxygen is added to an organic silane type gas such as ethyl orthosilicate to decrease the carbon content, the step coverage is degraded and therefore voids, a break of a wiring line, etc. may occur, which also cause reduction in reliability and degradation in semiconductor characteristics. In addition, an oxide film is more likely contaminated by impurities such as alkali metals. Once introduced, impurities behave as operative ions (movable ions) in some cases.